I had a 2009 Pontiac Vibe that was AWD and it was a big disappointment. The AWD sucked because it was one of those "slip and grip" systems (from the Toyota Rav4), plus it gave the car no power and sucked gas.
If you were to put AWD on a Sonic, you'd best design one that will work with the car instead of slapping some other existing system on. And IMHO the best AWD systems out there are Subaru and Audi Quattro because they WORK.
AND, make the engine make at least 160hp so it doesn't let you down when you need it, ie passing and merging on a freeway!